Proximity image tube with bellows focussing structure

ABSTRACT

The image tube employs a novel combination of elements which allow for utilization of a highly efficient III-V transmission photocathode activated to achieve negative effective electron affinity. The construction of the tube is especially suited for the activation of photocathode to achieve negative effective electron affinity.

[451 July 8,1975

United States Patent [191 Butterwick 442 W 99O 4 NNH mnw ao 3 /33 3 .3 0 .5 n m n m .m

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Primary Examiner Robert Segal Attorney, Agent, or FirmG. H. Bruestle; R. J. Boivin [21] Appl. No.: 369,581

[57] ABSTRACT The image tube employs a novel combination of elements which allow for utilization of a highly efficient III-V transmission photocathode activated to achieve 1 m 94 35 .l 04 H1 .)& 1

mm H3 1 .l m H ofim 1" m r. m S in d L6 .1 MF 1] 18 55 negative effective electron affinity. The construction [56] References Cited of the tube is especially suited for the activation of photocathode to achieve negative effective electron W m H a 004 0 H3 mm smm E nm wm An WW Sn Ear mm Tmm SRF D wm "9 wow 6 Claims, 3 Drawing Figures COOLER /WINDOW HEATER WINDOW PROXIMITY IMAGE TUBE WITH BELLOWS FOCUSSING STRUCTURE BACKGROUND OF THE INVENTION The invention disclosed herein was made in the course of. or under, a contract or subcontract thereunder with the Department of the Army.

The present invention relates to image tubes and more particularly to a proximity focussed image tube having a III-V photocathode.

An image tube is a type of tube which employs a pho tocathode which is sensitive to radiation across a particular wavelength region. When exposed to such radiation, electrons are emitted from the photocathode and are caused to travel to a phosphor coated anode where they strike the phosphors giving off visible light. Image tubes are particularly useful in applications where invisible radiation is converted into visible radiation, such as infrared and X-ray applications.

A proximity focussed image tube is an image tube having an anode quite close to the photocathode. The close spacing of the anode to the cathode diminishes the probability of large lateral movement of electrons traveling from the photocathode to the anode. This means that no additional focusing elements are required to operate such a tube.

For a number of years, much effort has been spent to develop transmission-type III-V photocathodes for use in image tubes. Generally, such photocathodes have been built and activated in experimental vacuum chambers and experimental tube closures. Recent effort has been directed toward transferring activated cathodes to tube closures within vacuum chambers. Because of the many compromises in activation techniques due to system mechanics, cathode problems such as low sensitivity and poor life have existed. Tubes having transfer cathodes have the additional problem of high cost due to the number and type of steps used in their manufacture.

SUMMARY OF THE INVENTION A proximity focussed image tube is presented which comprises a cylindrical vacuum-containing body, a transparent, heat conducting input window mounted on one end of the body; a semiconductor photocathode mounted within the body in contact with the input window, the semiconductor photocathode being activated to achieve negative effective electron affinity by means of an activation layer thereon; an output screen having a surface interior to the body adjacent the photocathode, the surface having a phosphor coating thereon; a bellows system sealing the output screen to the body whereby the surface of the output screen having the phosphor coating can be moved relative to the photocathode without disturbing the vacuum within the body; and means for making electrical contact to the photocathode and to the phosphor coating.

Also presented is the method of applying an activation layer to achieve negative effective electron affinity to a semiconductor layer within a vacuum tube and in contact with a heat conductive body extending out of the tube comprising the steps of heating the heat conductive body in contact with the semiconductor layer; then causing the semiconductor layer to be cooler than a second region within the tube; and diffusing cesium vapors into the second region whereby the vapors will diffuse to and condense upon the cooler semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a cross-sectional view of one embodiment of the image tube of the present invention;

FIG. 2 is a schematic representation of one step in the method of making the image tube; and

FIG. 3 is a schematic representation of another step in the method of making the image tube.

DETAILED DESCRIPTION Referring generally to FIG. 1, the preferred embodiment of the proximity-focussed image tube of the present invention is shown. The tube 10 comprises a vacuum-containing envelope 12 which is substantially cylindrical in shape. The cylindrical wall of the envelope l2 is comprised of a metallic window mount flange 9 sealed to a ceramic ring 11. The ceramic ring 11 is sealed to an annular member 21 which makes up a portion of the baffle system 22 of the tube, as will be explained hereinafter. The periphery of the annular member 21 is sealed to another ceramic ring 13, which in turn is sealed to a substantially cylindrical, metallic outer baffle wall 15 which makes up the outer wall of the baffle system 22 as will be further explained. The outer baffle wall 15 is attached to a metallic bellows having at its other end a metallic screen mount flange 25.

The tube 10 further comprises an input window 14 sealed at its periphery to an opening in the window mount flange 9. A semiconductor transmission photocathode 16 is mounted inside the envelope 12 in contact with the input window 14. The photocathode 16 is held in the opening in the window mount flange 9 by a washer 27 which is attached to the inside of the window mount flange 9. In the preferred embodiment of the image tube 10, the III-V semiconductor Indium-Gallium-Arsenide, InGaAs, (l8% lnAs; 82% GaAs), is used for the photocathode 16. This material is useful in converting [.06v umradiation into electron emission. However, other materials could be used for the photocathode 16 for conversion of radiation having other wavelengths. The photocathode 16 has an activation layer on the side opposite the input window 14, to achieve negative effective electron affinity (hereinafter NEA).

A fiber optic output window.l8 is sealed to the window mount flange at the end of the envelope 12 opposite the input window 14. The fiber optic output window 18 comprises a cylindrical body of fiber optic elements fused together with their longitudinal axes parallel to one another and perpendicular to a viewing surface 19 exterior to the tube 10. A thin, conductive, metallic anode 28 is deposited on the cylindrical surface of the fiber optic output window 18. A phosphor coating is deposited on the interior surface of the fiber optic window 18 opposite the viewing surface 19. In the preferred embodiment, P-ZO is used for this coating 30. An aluminum layer 29 having a thickness of about 1000 A is deposited on the phosphor coating 30 and makes up a portion of the anode 28. The aluminum layer 29 prevents light feedback to the photocathode l6, enhances the efficiency of the phosphor coating 30, and prevents contamination of the phosphor coating 30 by cesium.

The metallic bellows 20 which joins the screen mount flange 25 to the baffle wall 15 may be compressed as shown in FIG. 1 or expanded in order to move the phosphor coating 30 away from the photocathode 16.

The baffle system 22 surrounds the fiber optic window 18. The baffle system 22 includes an input tube 24 and an exhaust tube 26 which are hermetically sealed to the outer baffle wall 15. The annular member 21 is shaped to surround a baffle chamber 42. The input tube 24 opens into the chamber 42 formed between the annular member 21 and the outer baffle wall 15. The exhaust tube 26 extends through the chamber 42 and the annular member 21 and opens interior the tube adjacent the side wall of the output screen 18.

The length of that portion of the annular member 21 parallel to the outer baffle wall is such that a space 23 exists between the end of the annular member 21 and the bellows 20. This space 23 provides for entry of vapors from the baffle chamber 42 to the interior of the tube 10.

The metallic window mount flange 9 is used as the cathode electrode to make electrical contact to the photocathode 16. The screen mount flange 25 is used as the anode terminal of the tube as it is electrically connected to the anode coating 28 deposited on the fiber optics output window 18.

In the operation of the image tube 10, a voltage is ap plied to the anode 28 so it is positive with respect to the photocathode 16. Photoelectrons emitted from the photocathode 16, due to the presence of radiation transmitted through the input window 14, accelerate in the electric field between the photocathode 16 and the anode 28. Such electrons strike the phosphor coating 30 causing it to give off visible light which can be viewed at the fiber optic output window 18 on the viewing surface 19 exterior to the envelope 12.

Referring generally to FIGS. 1 and 2, the tube 10 of the present invention employs a novel combination of elements directed toward allowing for the manufacture of a highly efficient image tube employing a III-V photocathode activated to achieve NEA. In manufacturing the image tube 10, it is necessary to heat the photocathode 16 in order to prepare the surface for NEA activation. The input window 14 is made of a transparent material having a very high melting point and capable of sustaining a vacuum, such as sapphire. This is necessary because III-V photocathodes must be heated in the range of 500C in order to be properly prepared for NEA activation. By mounting the photocathode 16 on the input window 14, the photocathode 16 is heated by conduction whenever the input window 14 is heated. A contact heater 32 capable of heating the input window 14 above 500C is brought into intimate contact with the input window 14. The photocathode 16 is thereby heated in the range of 500C which accomplishes the dual functions of preparing the surface for NEA activation and secondarily, heating the tube envelope 12 beyond its normal bakeout temperature. This effectively outgasses the tube to a high degree. The bellows located between the envelope 12 and the output screen 18 provides an efficient stress isolation element and radiator which effectively reduces the temperature and mechanical stress on the fiber optic output screen as sembly 18.

Following the heat treatment, the photocathode 16 is ready for NEA activation. Referring generally to FIGS. 1 and 3, the novel combination employed in the image tube 10 allows a cathode cooler 34 to be brought into intimate contact with the input window 14, thereby cooling the photocathode 16 by conduction. At the same time that the cathode cooler 34 is used to cool the input window 14, a baffle heater 36 is brought into contact with the baffle assembly 22. The combination of the baffle heater 36 and the cathode cooler 34 provides a hot-cold thermal gradient within the tube 10. In addition, the heated baffle assembly 22 causes quick diffusion of cesium uniformly over the cold surface of the photocathode 16 when cesium vapors are introduced into the input tube 24.

The bellows 20 is expanded to move the fiber optic screen 18 away from the photocathode 16. Then, the process of activating the photocathode to achieve NEA is accomplished. The uniform distribution of cesium is performed by the baffle assembly 22 due to the combination of the annular member 21 and the opening 23 which cause the cesium vapors to be distributed throughout that portion of the baffle assembly between the annular member 21 and the wall 12 of the image tube 10. The vapors within the interior of the baffle system 22 diffuse through the opening 23 and thence onto the photocathode 16 due to the thermal gradient which has been established. Very little cesium will deposit upon the warmer surfaces of the tube 10. This is desirable as cesium deposited within the tube 10 on surfaces other than the photocathode 16 would cause unwanted current leakage when the tube 10 is operated. The purpose of the exhaust tube 26 is to allow for a cesium vapor equilibrium to be established within the tube 10, as well as for introducing oxygen into the tube 10. The oxygen acts with the cesium in activating the photocathode 16 to achieve NEA.

In addition to providing an effective cathode processing module, the tube 10 is compact, rugged, and versatile. The envelope 12 is comprised of a ceramic and metal structure which is very strong and free of vibration. The fiber optic output window 18 is an efficient and effective means for coupling the light output from the phosphor screen 30 to the viewing surface 19. The fiber optic window 18 has the function of transferring images from the plane of the phosphor coating 30 to the plane of the viewing surface 19. This means that contact prints can be made by putting photographic film directly on the viewing surface 19. In addition, other optical elements such as image intensifiers or video camera tubes can be coupled to the image tube 10 directly.

I claim:

1. A proximity focussed image tube comprising:

a. a vacuum containing body including:

1. a transparent heat conducting input window sealed to one end of said body;

2. an output screen sealed to another opposed end of said body;

3. a vacuum containing envelope including at least one tubular portion sealed between the opposed ends of said body which include said input window and output screen; said envelope being sealed to said input window and output screen by flanged portion which extend from opposed ends of said envelope and enclosing therewith an evacuated interior which extends between said opposed ends of said body;

b. a semiconductor photocathode mounted within said evacuated interior; said photocathode having a surface in contact with said input window;

c. a phosphor coating within the evacuated interior of said envelope on a surface of said output screen;

d. a bellows system including an expandable vacuum containing bellows having opposed ends which are sealed to portions of said envelope between said photocathode and said output screen whereby said surface of said output screen having said phosphor coating may be moved from a fixed proximity focussed position adjacent said photocathode to a position further removed from said photocathode by expansion of said bellows system, the expansion of said bellows system providing thermal and stress isolation of said output screen from said photocathode;

e. a chamber forming baffle system within the evacuated interior of said envelope whereby said photocathode may be uniformly exposed to vapors introduced into said system, said system including:

1. a baffle member extending into the evacuated interior of said envelope from a portion of the envelope; said baffle member forming a baffle chamber within the evacuated interior of said envelope substantially enclosed by surface portions of: said envelope, said output screen, and said baffle member;

2. an input tube extending from an outer portion of said envelope, and including a vapor passage communicating with and opening into said baffle chamber for introducing vapors into said system;

3. an exhaust tube extending from an outer wall portion of said envelope and including a vapor passage communicating with and opening into a portion of the evacuated interior, other than said baffle chamber, in which a surface of said photocathode is exposed; and

4. a vapor passage between said baffle chamber and said portion of said evacuatedinterior other than said baffle chamber, whereby vapor communication through and between said input and exhaust tubes may be accomplished.

2. The image tube of claim 1, wherein said input window is made of sapphire.

3. The image tube of claim 2, wherein said semiconductor photocathode is indium-gallium-arsenide.

4. The image tube of claim 1, wherein said envelope includes at least one cylindrical ceramic ring.

5. The image tube of claim 4, wherein said bellows system further comprises a cylindrical metallic bellows and wherein said bellows system is expandable upon substantial equalization of the pressure within the interior of said envelope in relation to the pressure externally applied to said body.

6. The image tube of claim 1, wherein said output screen additionally comprises a fiber optic member having a portion extending within the evacuated interior of said envelope, said portion being surrounded by a portion of said baffle member, and by said baffle chamber. 

1. A proximity focussed image tube comprising: a. a vacuum containing body including:
 1. a transparent heat conducting input window sealed to one end of said body;
 2. an output screen sealed to another opposed end of said body;
 3. a vacuum containing envelope including at least one tubular portion sealed between the opposed ends of said body which include said input window and output screen; said envelope being sealed to said input window and output screen by flanged portion which extend from opposed ends of said envelope and enclosing therewith an evacuated interior which extends between said opposed ends of said body; b. a semiconductor photocathode mounted within said evacuated interior; said photocathode having a surface in contact with said input window; c. a phosphor coating within the evacuated interior of said envelope on a surface of said output screen; d. a bellows system including an expandable vacuum containing bellows having opposed ends which are sealed to portions of said envelope between said photocathode and said output screen whereby said surface of said output screen having said phosphor coating may be moved from a fixed proximity focussed position adjacent said photocathode to a position further removed from said photocathode by expansion of said bellows system, the expansion of said bellows system providing thermal and stress isolation of said output screen from said photocathode; e. a chamber forming baffle system within the evacuated interior of said envelope whereby said photocathode may be uniformly exposed to vapors introduced into said system, said system including:
 1. a baffle member extending into the evacuated interior of said envelope from a portion of the envelope; said baffle member forming a baffle chamber within the evacuated interior of said envelope substantially enclosed by surface portions of: said envelope, said output screen, and said baffle member;
 2. an input tube extending from an outer portion of said envelope, and including a vapor passage communicating with and opening into said baffle chamber for introducing vapors into said system;
 3. an exhaust tube extending from an outer wall portion of said envelope and including a vapor passage communicating with and opening into a portion of the evacuated interior, other than said baffle chamber, in which a surface of said photocathode is exposed; and
 4. a vapor passage between said baffle chamber and said portion of said evacuated interior other than said baffle chamber, whereby vapor communication through and between said input and exhaust tubes may be accomplished.
 2. an output screen sealed to another opposed end of said body;
 2. The image tube of claim 1, wherein said input window is made of sapphire.
 2. an input tube extending from an outer portion of said envelope, and including a vapor passage communicating with and opening into said baffle chamber for introducing vapors into said system;
 3. an exhaust tube extending from an outer wall portion of said envelope and including a vapor passage communicating with and opening into a portion of the evacuated interior, other than said baffle chamber, in which a surface of said photocathode is exposed; and
 3. The image tube of claim 2, wherein said semiconductor photocathode is indium-gallium-arsenide.
 3. a vacuum containing envelope including at least one tubular portion sealed between the opposed ends of said body which include said input window and output screen; said envelope being sealed to said input window and output screen by flanged portion which extend from opposed ends of said envelope and enclosing therewith an evacuated interior which extends between said opposed ends of said body; b. a semiconductor photocathode mounted within said evacuated interior; said photocathode having a surface in contact with said input window; c. a phosphor coating within the evacuated interior of said envelope on a surface of said output screen; d. a bellows system including an expandable vacuum containing bellows having opposed ends which are sealed to portions of said envelope between said photocathode and said output screen whereby said surface of said output screen having said phosphor coating may be moved from a fixed proximity focussed position adjacent said photocathode to a position further removed from said photocathode by expansion of said bellows system, the expansion of said bellows system providing thermal and stress isolation of said output screen from said photocathode; e. a chamber forming baffle system within the evacuated interior of said envelope whereby said photocathode may be uniformly exposed to vapors introduced into said system, said system including:
 4. a vapor passage between said baffle chamber and said portion of said evacuated interior other than said baffle chamber, whereby vapor communication through and between said input and exhaust tubes may be accomplished.
 4. The image tube of claim 1, wherein said envelope includes at least one cylindrical ceramic ring.
 5. The image tube of claim 4, wherein said bellows system further comprises a cylindrical metallic bellows and wherein said bellows system is expandable upon substantial equalization of the pressure within the interior of said envelope in relation to the pressure externally applied to said body.
 6. The image tube of claim 1, wherein said output screen additionally comprises a fiber optic member having a portion extending within the evacuated interior of said envelope, said portion being surrounded by a portion of said baffle member, and by said baffle chamber. 